chore: import upstream snapshot with attribution

Sources/ContainerizationOCI/Descriptor.swift

@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025-2026 Apple Inc. and the Containerization project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+//  Source: https://github.com/opencontainers/image-spec/blob/main/specs-go/v1/descriptor.go
+
+import Foundation
+
+/// Descriptor describes the disposition of targeted content.
+/// This structure provides `application/vnd.oci.descriptor.v1+json` mediatype
+/// when marshalled to JSON.
+public struct Descriptor: Codable, Sendable, Equatable {
+    /// mediaType is the media type of the object this schema refers to.
+    public let mediaType: String
+
+    /// digest is the digest of the targeted content.
+    public let digest: String
+
+    /// size specifies the size in bytes of the blob.
+    public let size: Int64
+
+    /// urls specifies a list of URLs from which this object MAY be downloaded.
+    public let urls: [String]?
+
+    /// annotations contains arbitrary metadata relating to the targeted content.
+    public var annotations: [String: String]?
+
+    /// platform describes the platform which the image in the manifest runs on.
+    ///
+    /// This should only be used when referring to a manifest.
+    public var platform: Platform?
+
+    /// artifactType specifies the IANA media type of the artifact.
+    ///
+    /// Used in referrers API responses to indicate the type of each referring artifact.
+    public let artifactType: String?
+
+    public init(
+        mediaType: String, digest: String, size: Int64, urls: [String]? = nil, annotations: [String: String]? = nil,
+        platform: Platform? = nil, artifactType: String? = nil
+    ) {
+        self.mediaType = mediaType
+        self.digest = digest
+        self.size = size
+        self.urls = urls
+        self.annotations = annotations
+        self.platform = platform
+        self.artifactType = artifactType
+    }
+}